Fracture toughness measurements on igneous rocks using a high-pressure, high-temperature rock fracture mechanics cell

A sound knowledge of mechanical properties of rocks at high temperatures and pressures is essential for modelling volcanological problems such as fracture of lava flows and dike emplacement. In particular, fracture toughness is a scale-invariant material property of a rock that describes its resistance to tensile failure. A new fracture mechanics apparatus has been constructed enabling fracture toughness measurements on large (60 mm diameter) rock core samples at temperatures up to 750–C and pressures up to 50 MPa. We present a full description of this apparatus and, by plotting fracture resistance as a function of crack length, show that the size of the samples is sufficient for reliable fracture toughness measurements. A series of tests on Icelandic, Vesuvian and Etnean basalts at temperatures from 30 to 600–C and confining pressures up to 30 MPa gave fracture toughness values between 1.4 and 3.8 MPa m1=2. The Icelandic basalt is the strongest material and the Etnean material sampled from the surface crust of a lava flow the weakest. Increasing temperature does not greatly affect the fracture toughness of the Etnean or Vesuvian material but the Icelandic samples showed a marked increase in toughness at around 150–C, followed by a return to ambient toughness levels. This material also became tougher under moderate confining pressure but the other two materials showed little change in toughness. We describe in terms of fracture mechanics probable causes for the changes in fracture toughness and compare our experimental results with values obtained from dike propagation modelling found in the literature

Extreme events and predictability of catastrophic failure in composite materials and in the Earth

Despite all attempts to isolate and predict extreme earthquakes, these nearly always occur without obvious warning in real time: fully deterministic earthquake prediction is very much a ‘black swan’. On the other hand engineering-scale samples of rocks and other composite materials often show clear precursors to dynamic failure under controlled conditions in the laboratory, and successful evacuations have occurred before several volcanic eruptions. This may be because extreme earthquakes are not statistically special, being an emergent property of the process of dynamic rupture. Nevertheless, probabilistic forecasting of event rate above a given size, based on the tendency of earthquakes to cluster in space and time, can have significant skill compared to say random failure, even in real-time mode. We address several questions in this debate, using examples from the Earth (earthquakes, volcanoes) and the laboratory, including the following. How can we identify ‘characteristic’ events, i.e. beyond the power law, in model selection (do dragon-kings exist)? How do we discriminate quantitatively between stationary and non-stationary hazard models (is a dragon likely to come soon)? Does the system size (the size of the dragon’s domain) matter? Are there localising signals of imminent catastrophic failure we may not be able to access (is the dragon effectively invisible on approach)? We focus on the effect of sampling effects and statistical uncertainty in the identification of extreme events and their predictability, and highlight the strong influence of scaling in space and time as an outstanding issue to be addressed by quantitative studies, experimentation and models

The consolidation and bond strength of rafted sea ice

The consolidation and bond strength of rafted sea ice were investigated through a series of experiments undertaken in the Ice Physics Laboratory at the UCL. To simulate a section of rafted sea ice, blocks of laboratory grown saline ice were stacked in an insulated tank with spacers between adjacent blocks to allow saline water to flood in. The rate of consolidation was then monitored using a combination of temperature readings recorded in the ice and liquid layer, salinity measurements of the liquid layer, and cores taken at specific times of interest. Two states of consolidation were observed: thermodynamic consolidation where the ice blocks were physically bonded but the bond strength was weak, and mechanical consolidation where the bond had reached full strength. Results showed that the rafted ice had physically bonded in less than a day, however it took many more days (6 to 30 depending on the environmental conditions) for the bond to reach maximum strength. Increasing the thickness of the ice, the salinity of the water and the inter-block gap size all increased the consolidation time. Once consolidated, ice cores were taken and sheared using the asymmetric four-point bending method to measure the strength of the bond between the ice blocks. These were then compared to the shear strength of solid ice blocks simulating level sea ice. Our results show that the shear strength of the bond between the rafted ice blocks is about 30% weaker than that of level ice

Comparison of earthquake strains over 10(2) and 10(4) year timescales: insights into variability in the seismic cycle in the central Apennines, Italy

In order to study the existence of possible deficits or surpluses of geodetic and earthquake strain in the Lazio-Abruzzo region of the central Apennines compared to 15 +/- 3 kyrs multi seismic cycle strain-rates, horizontal strain-rates are calculated in 5 km x 5 km and 20 km x 20 km grid squares using slip-vectors from striated faults and offsets of Late Pleistocene-Holocene landforms and sediments. Strain-rates calculated over 15 +/- 3 kyrs within 5 km x 5 km grid squares vary from zero up to 2.34 +/- 0.54 x 10(-7) yr(-1) and resolve variations in strain orientations and magnitudes along the strike of individual faults. Surface strain-rates over a time period of 15 +/- 3 kyrs from 5 km x 5 km grid squares integrated over an area of 80 km x 160 km shows the horizontal strain-rate of the central Apennines is 1.18(-0.04)(+0.12)x10(-8) yr(-1) and -1.83(-4.43)(+3.80) x 10(-10) yr(-1) parallel and perpendicular to the regional principal strain direction (043 degrees-223 degrees+/-1 degrees), associated with extension rates of <= 3.1(-0.4)(+0.7) mm yr-1 if calculated in 5 km x 80 km boxes crossing the strike of the central Apennines. These strain-rates are similar in direction to strain-rates calculated using geodesy (over 126 yrs, 11 yrs and 5 yrs) and seismic moment summation (over 700 yrs); however, the magnitude is about 2.6 x less over a comparable area. The 10(2) yr strain-rates are higher than 10(4) yr strain-rates in some smaller areas (approximate to 2000 km(2), corresponding to polygons defined by geodesy campaigns and seismic moment summations) with the opposite situation in other areas where seismic moment release rates in large (>Ms 6.0) magnitude historical earthquakes have been reported to be as low as zero. This demonstrates the importance of comparing the exact same areas and that strain-rates vary spatially on the length scale of individual faults and on a timescale between 10(2) yr and 10(4) yr in the central Apennines. We use these results to produce a fault specific earthquake recurrence interval map and discuss the regional deformation related to plate boundary and sub-crustal forces, temporal earthquake clustering and the natural variability of the seismic cycle

Damage of saturated rocks undergoing triaxial deformation using complex electrical conductivity measurements: Experimental results

The frequency dependent complex electrical conductivity of brine saturated rocks is extremely sensitive to changes in the volume, connectivity, orientation, and surface topography of pores and cracks. We have made triaxial deformation experiments on sandstone specimens saturated with distilled water. Experiments were carried out for several values of confining pressure, and in both drained and undrained regimes. During the deformation the full complex (in-phase and out-of-phase) electrical parameter set was measured (i.e. conductivity, resistivity, permittivity etc.) for 50 frequencies from 20 Hz to 1 MHz. Only the data at 1 kHz will be discussed here. This data tracks how the rock undergoes crack closure, followed by dilatancy, crack linking, and finally failure, as axial strain is increased. The data indicates well how early the formation of new cracks begins, showing that the quasi-linear portion of the stress-strain curve for triaxial deformation of saturated rocks does not represent truly elastic behaviour, but represents the combined effects of crack closure perpendicular to the strain axis and the formation of tensile cracks parallel to the strain axis. The electrical data has also been used to derive an electrical-equivalent change in porosity, and to examine the way that the cementation exponent and the tortuosity of the pore and crack network change during deformation. © 1997 Published by Elsevier Science Ltd

A 667-year record of co-seismic and interseismic Coulomb stress changes in central Italy reveals the role of fault interaction in controlling irregular earthquake recurrence intervals

Current studies of fault interaction lack sufficiently long earthquake records and measurements of fault slip rates over multiple seismic cycles to fully investigate the effects of interseismic loading and coseismic stress changes on the surrounding fault network. We model elastic interactions between 97 faults from 30 earthquakes since 1349 A.D. in central Italy to investigate the relative importance of co-seismic stress changes versus interseismic stress accumulation for earthquake occurrence and fault interaction. This region has an exceptionally long, 667 year record of historical earthquakes and detailed constraints on the locations and slip rates of its active normal faults. Of 21 earthquakes since 1654, 20 events occurred on faults where combined coseismic and interseismic loading stresses were positive even though ~20% of all faults are in “stress shadows” at any one time. Furthermore, the Coulomb stress on the faults that experience earthquakes is statistically different from a random sequence of earthquakes in the region. We show how coseismic Coulomb stress changes can alter earthquake interevent times by ~103 years, and fault length controls the intensity of this effect. Static Coulomb stress changes cause greater interevent perturbations on shorter faults in areas characterized by lower strain (or slip) rates. The exceptional duration and number of earthquakes we model enable us to demonstrate the importance of combining long earthquake records with detailed knowledge of fault geometries, slip rates, and kinematics to understand the impact of stress changes in complex networks of active faults